Current state of the art refers to materials which change phase under the influence of light or electric field or current for use as non-volatile memory elements. These phase-change memories (also known as PCME, PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM and C-RAM) is a type of non-volatile computer memory. PRAMs exploit the unique behavior of chalcogenide glass. Heat produced by the passage of an electric current switches this material between two states, crystalline and amorphous. Recent versions can achieve two additional distinct states, in effect doubling their storage capacity. PRAM is one of several new memory technologies competing in the non-volatile role with the almost universal flash memory.
Stanford R. Ovshinsky of Energy Conversion Devices first explored the properties of chalcogenide glasses as a potential memory technology in 1960. In 1969, Charles Sie published a dissertation, [“Memory Devices Using Bistable Resistivity in Amorphous As—Te—Ge Films” C. H. Sie, PhD dissertation, Iowa State University, Proquest/UMI publication #69-20670, January 1969] [“Chalcogenide Glass Bistable Resistivity Memory” C. H. Sie, A. V. Pohm, P. Uttecht, A. Kao and R. Agrawal, IEEE, MAG-6, 592, September 1970] that both described and demonstrated the feasibility of a phase change memory device by integrating a chalcogenide film with a diode array. A study in 1970 established that the phase change memory mechanism in chalcogenide glass involves electric-field-induced crystalline filament growth. [“Electric-Field Induced Filament Formation in As—Te—Ge Semiconductor” C. H. Sie, R. Uttecht, H. Stevenson, J. D. Griener and K. Raghavan, Journal of Non-Crystalline Solids, 2, 358-370, 1970] In the September 1970 issue of Electronics, Gordon Moore published an article on the technology. However, material quality and power consumption issues prevented commercialization of the technology.
The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity. The amorphous, high resistance state represents a binary 0, while the crystalline, low resistance state represents a 1. Chalcogenide is the same material used in re-writable optical media (such as CD-RW and DVD-RW). In those instances, the material's optical properties are manipulated, rather than its electrical resistivity, as chalcogenide's refractive index also changes with the state of the material.
Wuttig and Yamanda Nat Mat 6, 824 (2007), Sokolowski-Tinten PRL 81, 3679 (1998), Siegel APL 84, 2250 (2004), Forst App. Phys. Lett. 77, 1964 (2000) report changes of the coherent phonon spectrum associated with the transition. Rueda APL 98 251906 (2011) discuss the difference in vibrational frequencies in amorphous and crystalline phases. The mechanism for switching is discussed by Kolobo et al Nat. mat 3, 703 (2004) who calculated an umbrella flip transition of Ge atoms responsible for switching between the amorphous and crystalline state. The present state of phase change memories was reviewed by Burr et al. (Journal of Vacuum Science and Technology B, 28, pp. 223-262, (2010)), listing pertinent patents and other publications.
Although PRAM has not yet reached the commercialization stage for consumer electronic devices, nearly all prototype devices make use of a chalcogenide alloy of germanium, antimony and tellurium (GeSbTe) called GST. The stoichiometry or Ge:Sb:Te element ratio is 2:2:5. When GST is heated to a high temperature (over 600° C.), its chalcogenide crystallinity is lost. Once cooled, it is frozen into an amorphous glass-like state and its electrical resistance is high. By heating the chalcogenide to a temperature above its crystallization point, but below the melting point, it will transform into a crystalline state with a much lower resistance. The time to complete this phase transition is temperature-dependent. Cooler portions of the chalcogenide take longer to crystallize, and overheated portions may be remelted. A crystallization time scale on the order of 100 ns is commonly found. [H. Horii et al., 2003 Symposium on VLSI Technology, 177-178 (2003).] This is longer than conventional volatile memory devices like modern DRAM, which have a switching time on the order of two nanoseconds. However, a January 2006 Samsung Electronics patent application indicates PRAM may achieve switching times as fast as five nanoseconds.
A more recent advance pioneered by Intel and ST Microelectronics allows the material state to be more carefully controlled, allowing it to be transformed into one of four distinct states; the previous amorphous or crystalline states, along with two new partially crystalline ones. Each of these states has different electrical properties that can be measured during reads, allowing a single cell to represent two bits, doubling memory density. [A Memory Breakthrough, Kate Greene, Technology Review, 4 Feb. 2008].
de Jong et al. (Phys. Rev. Lett. 108 157601 (2012)) report on magnetic non-volatile memory switching by laser-excited rare-earth orthoferrite (SmPr)FeO3 demonstrating that a single 60 fs circularly polarized laser pulse is capable of creating a magnetic domain structure on a picosecond time scale with a magnetization direction determined by the helicity of light. Kirilyuk, Kimel and Rhasing (Rev. Mod. Phys. 82, (2010)) review the ultrafast switching of domain structures by laser pulses in different magnetic systems. The important role played by the magnetic domain wall dynamics in the process of the laser-induced magnetization reversal has recently been demonstrated using time-resolved magneto-optical imaging of laser-excited TbFeCo thin films Ogasawara et al., 2009.
None of the published works on magnetic systems discuss electronic or structural domain formation, or a quench mechanism for the formation of magnetic or any other kinds of domains.
Rini et al (Optics letters 30, 558 (2005)) describe an ultrafast switching device based on VO2 which relies on switching of a first order structural transition, but does not involve domain formation by ultrafast quench, and the same transition can be achieved by slow thermal cycling.
Memristor memory recently developed by Hewlett Packard in 2008 [HP and Hynix to produce the memristor goods by 2013, PCs and Chips, 10 Oct. 2011] is based on the nonlinear resistance properties of metal-oxide-metal junctions, such that when current flows in one direction through a memristor, the electrical resistance increases; and when current flows in the opposite direction, the resistance decreases (“Mem ristor FAQ”. Hewlett-Packard. http://www.hpl.hp.com/news/2008/apr-jun/memristor_faq. html Retrieved 2010 Sep. 3.).
Kahn (Appl. Phys. Lett., vol. 22, p. 111 (1973), U.S. Pat. No. 4,405,993, Sep. 20, 1983) reported on a memory device in which cooling of a (preferably) smectic liquid crystal display is used to create a strongly scattering and depolarizing smectic texture, whereby rapid cooling freezes in disorder present in the isotropic state, while slow cooling permits the disordered molecules to reorganize into the uniform nonscattering structure favored by the boundary conditions and be used to erase written information. The spot size of the scattering region is comparable to the dimensions of the laser beam, and according to the patent, the structure is stable in a temperature range between −10 and +40 C, which may be extended by special mixtures.
Yusupov et al (Nat. Phys. 6, 681 (2010)) discuss the formation of domains in TbTe3 and related compounds DyTe3, K0.3MoO3 and TaSe2, created by an ultrafast quench mechanism and their annihilation on the picosecond timescale. The authors report on the creation of a domain structure with domains parallel to the surface of the crystal due to the inhomogeneous nature of the excitation and because of the fact that the coherence length of the order parameter in the material is much smaller than the photoexcited region, topological defects are created. They do not report on any metastable or persistent states in these compounds caused by the quench.
The technical problem which has not been solved according to the background prior art described above is solved by the current invention of an ultrafast quench based nonvolatile bistable device. Here the term “quench” is understood as defined by the Oxford English dictionary, namely as “rapidly cool”.